Hematophagous arthropods transmit pathogens of medical and veterinary public health importance, and many of these disease causing agents undergo complex developmental cycles in the vector. Ixodid ticks are the most important arthropod vectors of infectious agents to domestic livestock and wild animals, and they are second to mosquitoes as transmitters of pathogens to humans.
Control of ticks throughout the world is dependent upon acaricides, and broad ranging resistance to these chemicals represents an extremely significant threat to animal health and production, resulting in economic losses to producers and major reductions in food supplies. Acaricide resistance is a global problem involving single and multiple host ticks, and varying degrees of resistance exist for all major chemical groups used for ixodid control.
There are numerous problems associated with chemical control of arthropods. Those include impact of the chemicals on the environment, damage to non-target species and their presence as meat and milk residues. Many of the problems give rise to possible human health implications. These considerations, combined with heightened awareness of an increasing incidence and variety of tick transmitted diseases indicate the need for alternative tick control strategies.
Tick saliva contains pharmacologically active substances and a number of immunogens. Engorgement, that is feeding on host blood, might occur over a period of four to fifteen days. An unfed ixodid female, with a final fed weight of 1000 milligrams, might process 4000 milligrams of host blood, alternating between uptake of blood and salivation every five to 30 seconds. The host can be exposed to a significant quantity and array of immunogenic tick molecules.
Cattle and laboratory animal species acquire resistance to a variety of ixodid species as a consequence of infestation. Impact of acquired resistance is expressed by reduced number of engorged ticks, decreased volume of blood meal, reduced number and viability of ova, increased feeding time, tick death and impaired transmission of pathogens. Cell mediated, antibody and complement dependent effector mechanisms are all part of the complex immunological basis of acquired tick resistance.
Development of immunization induced anti-tick immunity is a logical extension of studies designed to characterize acquired resistance. The first vaccination attempt used an extract of whole Dermacentor variabilis larvae as an antigen source (Trager, W., "Acquired Immunity To Ticks," Journal of Parasitology, 25:57-81 (1939)). Salivary gland extracts have been used as immunogens to induce host anti-tick immunity that resembled acquired resistance to Ixodes holocyclus (Bagnall, B. G., "Cutaneous Immunity To the Ixodid Tick Ixodes holocyclus," Ph.D. Thesis, University of Sydney, 186 pp. (1975)), Boophilus microplus (Brossard, M., "Relations immunologiques entre bovins et tiques, plus partialierement entre bovins et Boophilus microplus, Acta Tropica 33:15-36 (1976)); and Dermacentor andersoni (Wikel, S. K., "The Induction of Host Resistance To Tick Infestation With a Salivary Gland Antigen", American Journal of Tropical Medicine and Hygiene, 30:284-288 (1981)). Immunization with salivary gland immunogens induced variable levels of host resistance and intense cutaneous reactions often developed at tick attachment sites (Wikel, S. K., "Immunological Control of Hematophagous Arthropod Vectors: Utilization of Novel Antigens", Veterinary Parasitology, 29:235-264 (1988)). An alternative source of immunogen, which was used to induce significant levels of anti-tick resistance, was primary tick tissue culture cells established from developing larvae (Wikel, S. K., "Resistance To Ixodid Tick Infestation Induced By Administration of Tick Tissue Culture Cells," Annals of Tropical Medicine and Parasitology, 79:513-518 (1985)). Vaccination with primary embryo culture cells of Amblyomma americanum stimulated significant levels of host immunity not only to A. americanum, but also to a challenge infestation with adult D. andersoni (Wikel, S. K. (1985) supra).
Recently, anti-tick vaccination studies have focused on "novel" immunogens thought not to be associated with acquired resistance, particularly ixodid digestive tract molecules (Willadsen, P., "Immunological Approaches To the Control of Ticks," International Journal for Parasitology, 17:671-677 (1987); Wikel, S. K., "Immunological Control of Hematophagous Arthropod Vectors: Utilization of Novel Antigens," Veterinary Parasitology, 29:235-264 (1988)). Reported studies involved development of anti-B. microplus (Willadsen, P., "Immunological Approaches To the Control of Ticks," International Journal for Parasitology, 17:671-677 (1987); Wong, J. Y. M. and Opdebeeck, J. P., "Protective Efficacy of Antigens Solubilized From Gut Membranes of the Cattle Tick, Boophilus microplus," Immunology, 66:149-155 (1989)) and anti-A. americanum (Wikel, S. K., "Immunological Control of Hematophagous Arthropod Vectors: Utilization of Novel Antigens," Veterinary Parasitology, 29:235-264 (1988)) vaccines.
Willadsen, P. et al., "Immunologic Control of a Parasitic Arthropod. Identification of a Protective Antigen From Boophilus microplus," Journal of Immunology, 143:1346:1351 (1989), using a carefully designed biochemical approach, identified a protection inducing 89.0 kilodalton, kdal., glycoprotein derived from B. microplus gut. Variable levels of host resistance were elicited by immunization with this molecule (Willadsen and Kemp, "Vaccination With `Concealed` Antigens For Tick Control," Parasitology Today, 4:196-198 (1988)). A second group investigating bovine resistance to B. microplus has reported 87 percent protection and a 95 percent reduction in ova production by administration of tick midgut (Opdebeeck et al., 1988). Protection was stimulated by administration of detergent solubilized immunogen, and magnitude of resistance expressed was correlated with bovine major histocompatibility antigens (Wong, J. Y. M. and Opdebeeck, J. P., "Protective Efficacy of Antigens Solubilized From Gut Membranes of the Cattle Tick, Boophilus microplus," Immunology, 66:149-155 (1989)). The nature of the composition of the protection inducing immunogen(s) has not been described.
Significant levels of resistance to infestation with adult A. americanum were induced with microgram quantities of tick gut enriched for brush border fragments, or the 27,000 g supernatant from which brush border fragments were separated (Wikel, S. K. "Immunological Control of Hematophagous Arthropod Vectors: Utilization of Novel Antigens," Veterinary Parasitology, 29:235-264 (1988)). Mean engorgement weights of ticks from immunized host groups were reduced by up to 69.8 percent, as many as 71.5 percent of challenge ticks were killed and up to 50 percent of viable females did not produce ova.
Sera produced from immunized and control animals were used to identify candidate immunogens by immunoblotting (Wikel, S. K., "Immunological Control of Hematophagous Arthropod Vectors: Utilization of Novel Antigens," Veterinary Parasitology, 29:235-264 (1988)). Lectins, peanut agglutinin and wheat germ agglutinin, and immunized host antibodies bound extract components with similar molecular weights on blots of one dimensional sodium dodecyl sulfate polyacrylamide gradient gels. These latter attempts as reported by the inventor in 1988 represent a step along the way; however, they reported less than satisfactory blood spectrum immunity and in some instances the vaccinated mammal showed an adverse skin response to tick feeding caused by secondary cutaneous infection.
To date, in spite of many attempts, there has not yet been developed a successful protocol method and vaccine for immunizing domestic animals against widespread tick infestation. As can be seen from the above description, there is a real and continuing need for a method of selecting and isolating in pure form useful immunogens, for development of protocols for extraction of successful immunogens, and for the development of vaccines containing those immunogens. This invention has as its primary objective the fulfilling of this need.